Long-range atmospheric transport of microplastics across the southern hemisphere

Airborne microplastics (MPs) can undergo long range transport to remote regions. Yet there is a large knowledge gap regarding the occurrence and burden of MPs in the marine boundary layer, which hampers comprehensive modelling of their global atmospheric transport. In particular, the transport efficiency of MPs with different sizes and morphologies remains uncertain. Here we show a hemispheric-scale analysis of airborne MPs along a cruise path from the mid-Northern Hemisphere to Antarctica. We present the inaugural measurements of MPs concentrations over the Southern Ocean and interior Antarctica and find that MPs fibers are transported more efficiently than MPs fragments along the transect, with the transport dynamics of MPs generally similar to those of non-plastic particles. Morphology is found to be the dominant factor influencing the hemispheric transport of MPs to remote Antarctic regions. This study underlines the importance of long-range atmospheric transport in MPs cycling dynamics in the environment.

Lines 84-85: The authors suggest that there is a decrease in MPs fragment concentration from north to south, as if there is a linear relationship between latitude and microplastic presence in the atmosphere (hypothesis also reinforced by the use of the linear regression analysis in figure 1) which do not seem to be justified.What is this hypothesis based on?
Lines 133-136: The back-trajectory analysis, which seems to carry an important message in this work (i.e. that the northern latitudes samples are related to a meaningful influence of land sources with respect to the southern ones), looks not very robust.My main concerns are the following: -it is not clear how the trajectories release points (i.e. the samples for which the transport analysis is shown) in figure S4 are chosen.Why not putting together the air masses transport analysis for the different latitude sections instead of doing the analysis only on 6 samples?-Panel B is cut on the northern latitudes and it is not really possible to see if the trajectories are actually reaching the land or not -The trajectory analysis does not seem to consider the vertical uplift of the air masses.That means that it is not considered if the air masses when traveling over land are reaching the surface or are above the planetary boundary layer.If the last condition is true, then the land sources may actually not have influenced the air masses and the MPs could come from the ocean instead.Also, in the description of the trajectories methods (lines 579,580) is not clear what is meant by "Back trajectories were run every 12 hours during the 7 days before the sampling date".That sounds incorrect, as there would be no reasons to release trajectories up to 7 days before the sampling date.Is 7 days the length of the trajectories computation back in time, or the length of the release time interval?This needs to be clarified and both quantities need to be specified.
Lines 149-152: This is another important concept that needs to be stated cautiously.The results from the study of Yang et al. 2022 are relative to specific sizes of fibers (~100 um length) and it has been performed in a tank experiment.While this is a starting point to hypothesize that fibers are ejected from water with less probability with respect to spheres, it is still not enough to assume safely that the long-range transport is the responsible for the fibers' presence observed in this study.Since the fibers you observe are quite large in diameter (order of tenths um) and length (order of hundreds um and some even thousands), even if the morphology surely affects their lifetime in the atmosphere with respect to fragments, one can also hypothesize that are also not being suspended since too long.Are the back trajectories originating from the fibers sampling always reaching land in a reasonable time frame?And how is this related to the number of fibers observed?This can be a way to check, in case you want to assume that they are only emitted on land, which would be the time of transport from there and if it is more reasonable to hypothesize that they are actually also emitted from the sea.
Lines 168-186: I think the analysis on the sealant tar is quite interesting, but I do wonder how the authors can exclude that this is not coming instead from ships coating (for example epoxy resins are widely used for this purpose) and therefore from ocean rather than the coast (or a mixture of the two).

Minor and technical comments:
Lines 61,62: In this sentence the authors seem to put together two concepts (the ocean winds/sea spray mist with the earth electric field forces) that I am not sure are really fitting with each other in the framework of that paragraph.If we want to emphasize the uncertainties related to the presence and transport of atmospheric microplastic in the general sense, at hemispheric level (as the paragraph seems to suggest), it is the identification of the sources, the mechanism of emission, resuspension and the transport and removal processes of MPs that constitute the overall uncertainty.While the electric forces may be indeed part of the general equation, the ocean winds and the sea spray represent only a small fraction of it when talking about the total sources.I guess the authors mean that, among all the uncertainties that are related to the MP transport in the atmosphere, the work is going to provide observational quantification of the presence of MP in the Southern hemisphere marine atmosphere, which will help understand better the role of the ocean as a source of MP in the bigger picture.If that is the case, I would encourage the authors to emphasize more clearly this point.Lines 144,145: This sentence is not clear, there may be a typo.Do you mean "Similar phenomena OF constant concentrations..."? Lines 184-186: Do you mean here that the dominance of white-transparent colors for fibers indicates they were subjected to longer aging (hence long-range transport) with respect to the colored fragment?It is not clear how you arrive to your conclusion, it will be useful if you could elaborate more explicitly on this point.
Line 399: I would suggest to explicitly say what the ANOVA analysis is, at least the first time you mention it.

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Response to Reviewers' Comments Dear Reviewers, We would like to thank you for the thoughtful and helpful comments on our manuscript.We have carefully addressed all comments and revised the manuscript accordingly.Below, we provide detailed responses to all your comments.The reviewers' comments are presented in bold Times New Roman italics font, and our response to the science-specific comments is written in normal font following each of the individual comments.Revisions to the manuscript are included directly in the response to reviewers document, where additions are marked in blue and deletions are struck out in red.In addition, all the line numbers in the response to reviewer documents refers to the "revised manuscript with marked changes".
We feel it is important to note that the reanalysis of 100% of the filters inside the MBL has now been performed and the results are included in the revised version of the manuscript, and the new data are in line with the previous analysis and support the main results.Specifically, our findings demonstrate that the migration of microplastic fibers is more efficient than microplastic fragments, as supported by the hemispheric measurements of airborne microplastics and predictive calculations from backtrajectory measurements (which have also been further developed through the revision process).This work highlights the migration of microplastics from land to the Southern Ocean, which will be of great interest to those working in the field.
Thank you for your time and consideration.

Best regards
Guitao Shi on behalf of all co-authors R2

Hemisphere" dealing with the atmospheric transport of microplastics to remote regions. The authors present a hemispheric-scale analysis of airborne microplastics taken during a cruise path. It is true that the transport efficiency of microplastics in the atmosphere is still poorly known, not only in the Southern
Hemisphere, but globally, and the data presented are interesting.However my feeling is that they are more routine then a real breakthrough: the results showed that fibers were more efficiently transported, which is a known fact.Besides, the discussion provided by the authors and the interpretation of data present some shortcomings.In include details below.

Response:
Thank you so much for pointing out the strengths of our study as well as offering suggestions for improving the manuscript's discussion and significance.The primary contributions of this study are as follows: (1) it is the first to report hemispheric transport of airborne microplastics, even though it utilizes well-established monitoring and characterization methods, (2) our findings demonstrate that the migration of microplastic fibers is more efficient than microplastic fragments, as supported by both practical measurements and predictive calculations, and (3) we present evidence of the migration of microplastics from land to the Southern Ocean, which is derived from our analysis of the trends in the weathering indices of certain types of microplastic samples.To make these unique aspects of our study more apparent to the reader, we have supplemented our initial submission with more in-depth discussions and improved the data interpretation quality carefully, as suggested by all reviewers.

Response:
Thank you very much for pointing this out.We have revised the expression in the manuscript: "For instance, airborne micro(nano)plastics (MnPs) monitored by current technology have a typical size range of 0.2-5000 µm 20 µm-5 mm, making them noticeably larger than the typical aerosols typically found in the atmosphere (e.g., sea salts) in the atmosphere."(Lines 53-57)

Response:
Thank you for expressing your concerns on this point.We completely agree with the perspectives you have shared.It is quite true that currently the detection of nanoplastics in environmental samples is limited, with only a few specific cases reported where high concentrations were observed.Therefore, we revised the expression as follows.

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"For instance, airborne micro(nano)plastics (MnPs) monitored by current technology have a typical size range of 0.2-5000 µm 20 µm-5 mm, making them noticeably larger than the typical aerosols typically found in the atmosphere (e.g., sea salts) in the atmosphere."(Lines 53-57)
Reviewer #1 Comment 4. The authors claim (several times) that they used a unified and standardized monitoring method, but this was because they were the results of a single expedition.It would have been strange that they didn't unify methods with themselves.Comparing data from different studies is not a weakness (line 82), but a strength because of result crosschecking.

Response:
With this statement we meant that comparing studies performed in different regions to understand the geographic distribution of airborne microplastics is challenging because all groups carry out microplastic sampling and analysis differently.The data analyzed in this study were obtained from a single expedition, and a large number of representative sampling points (26 points, spanning from latitude ~30°N to ~70°S) were assessed in the same manner.
To the best of our knowledge, this sample size, and particularly the latitudes which are covered, is larger than any previously reported voyages aiming to quantify atmospheric microplastics.We agree that comparing our results with other studies is beneficial despite the lack of a widely adopted sampling and analysis protocol.Consequently, we have included a new figure (Figure S3) and a new table (Table S3) regarding the atmospheric microplastics measured between previous studies and the current study.To make a reasonable comparison, especially in light of the reviewer's previous comments, references included microplastic studies and excluded nanoplastic studies.
We have also indicated the analytical method(s) which were used in each case, including the method size range, as well as the environment in which the samples were taken.We found that the airborne MPs concentrations detected in this study are not significantly different from previous studies sampled over the open ocean or coasts, but significantly lower than those collected over the land by orders of magnitude (p<0.05).
"The airborne MPs concentrations measured in this study (0.035 ± 0.009 n m -3 ) were not significantly different from previous studies sampled over the open ocean or coasts (2.12 ± 3.84 n m -3 ; p=0.243), but significantly lower than those collected over the land by orders of magnitude (24.77 ± 40.61 n m -3 ; p=0.040) (Table S3, Figure S3)." (Lines 121-125) "The analytical approach we used in this study is in line with approaches which are typically used for collection and characterization of airborne microplastics, despite there being no standardized methods across studies."(Lines 578-580)

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authors should discuss the details of the spectra in the region 3000-3800 cm -1 that support the attribution to rayon of a high number of fibres.

Response:
Thanks very much for this suggestion to point out these challenges to future readers.It is true that rayon (a man-made fiber) is challenging to distinguish from cotton or other cellulosic materials using spectroscopic methods.As this was also an issue that occurred during our field investigation and subsequent laboratory analysis, our group has developed a method to accurately distinguish rayon from cotton (Cai et al. 2019).The characteristic peak at 1105 cm -1 is very distinct for rayon, which can be applied to distinguish man-made rayon from natural cotton.Therefore, this is an important feature for polymer identification in this study.
Furthermore, we have also found another characteristic peak for distinguishing rayon of the broad and featureless shapes of the band at 3330 cm -1 according to (Comnea-Stancu et al. 2017).This band is the characteristic profile of the O-H stretching band.The shape of this band is rather broad and featureless for man-made fibers, whereas it exhibits a distinct maximum at ,,,)bDM _* for natural fibers.
Therefore, we can confidently distinguish rayon and cotton in this study by introducing the two methods described above.We have revised the manuscript as follows in the Methods section.
"Considering that the man-made fiber rayon is challenging to distinguish from cotton or other cellulosic materials, we have adopted two methods for its accurate identification.First, the characteristic peak at 1105 cm -1 is very distinct for rayon, and we have embedded this information into the spectra matching library according to our previous research (Cai et al. 2019).
Second, another characteristic band at 3330 cm -1 of the O-H stretching can also be utilized to distinguish rayon, where its

Response:
Thank you very much for your pertinent suggestions.Indeed, our use of the term "equivalent diameters" was not explained in detail in the previous version of the manuscript.As the reviewer correctly points out, ImageJ can only present the mean projected area for fragments and fibers.Therefore, after obtaining the width, length, and the calculated height parameters, we estimated the surface area and volume (mass) values of fibers and fragments with different models.For fibers, the cylinder model was applied according to Simon et al. (2018); for fragments, the column model was applied according to a previous study (Koelmans et al. 2020) together with an approximation by assuming the length-to-width ratio equates to the width-toheight ratio (L/W=W/H) (Mintenig et al. 2020, Simon et al. 2018).
For the fibers, we measured the projected width and length of particles utilizing ImageJ.The width was equivalent to the diameter of the bottom surface of a cylinder, the length was equivalent to the height of the cylinder, and we calculated the surface area according to Eq S1.For the fragments, we measured the projected length and width of particles utilizing ImageJ, projected the height using L/W=W/H, and calculated the surface area according to Eq S2.

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SAfiber = "'% # $ ) '% ( /2 (Eq.S1) where SA represents surface area, R is the diameter and L is the length obtained from ImageJ for fiber particles, W is the width and L is the length obtained from ImageJ for fragment particles.
Collectively, after obtaining the width and length parameters, we estimated the surface area of each particle (Figures 4 and   S12).The explanations detailed above were included in the revised Supporting Information (Text S3).
"We used the cylinder model (Simon et al. 2018) and the column model (Koelmans et al. 2020) to estimate the surface area and volume (mass) values of fibers and fragments (details see Text S3).Additionally, an approximation by assuming the length-to-width ratio equates to the width-to-height ratio (L/W=W/H) was also applied for fragments (Mintenig et al. 2020, Simon et al. 2018).However, it is important to note that this simplified approach to suggest surface area and volume (mass) likely represents a low estimate, as it does not take into account additional surface roughness and cracks which are likely to be present on environmental microplastics, subsequently increasing the actual surface area."(Lines 631-638)

Reviewer #1 Comment 8. What is the pore size of glass fibre filters and how the authors measured the flow rate? Didn't the flow rate change during sampling? This is the reason why sampled volumes are generally much lower that those
reported in this work.Sampling details are not sufficiently detailed in the manuscript.

Response:
=IBNKS VFRY MUDI GOR TIF DOMMFNTS' ?F IBVF SUPPLFMFNTFE TIF EFTBJLS BCOUT PORF SJZF OG TIF HLBSS GJCFR GJLTFRS "+'.`M#% flow rate (1.2 m 3 min -1 ), and sampling volume which are now included in the revised manuscript.Additional information on how the filters were placed, removed from the sampler and stored prior to analyses was also included.
"Atmospheric particles were collected onto Whatman quartz fiber filters (Whatman QM-A, 8×10 in, 20.3 cm × 25.4 cm, with TIF PORF SJZF OG +'.`M).At each site, the membranes were prebaked at ~500 o C for > 6 h to ensure the filters were free of polymer particles."(Lines 528-531) "For mounting the filters, the buckles of the upper placing plate were unscrewed after opening the upper cover of the HVAS, and then a filter membrane was laid flush on the lower plate.Then the buckles were re-tightened, and the upper cover of the HVAS was closed.The entire process was generally completed within about 30 s, reducing the chance of airborne contamination.Note that the filter has one rough surface and one smooth surface, and atmospheric particles were always collected on the rough surface.Typically, the pump of the high-volume air sampler (TISCH Environmental, USA) was running at a constant flow rate, 1.2 m 3 min -1 , and for a sampling duration of 48 h which would consequently lead to sampling a volume of 3456 m 3 .The flow rate of the pump was calibrated when manufactured in the factory.The airflow rate of the HVAS was relatively constant, at 1.2 m 3 min -1 , and a sampling duration of 2-3 days for each sample led to typical sampling air volumes of ~3000-4000 m 3 .Typically, aAtmospheric particles along the 2-3 days' cruise path, covering approximately 2-4 degrees of latitude, were collected on one filter which was considered one aggregate sample.A wind direction sensor was employed to control the HVAS to avoid potential contamination from the vessel, so consequently only air masses from a sector ~120 o left and right of the central line of the vessels' path was sampled.After sampling, the upper cover of the HVAS was opened and buckles of the upper placing plate were unscrewed, then the filter was removed by pre-cleaned stainless tweezers from the sampler."(Lines 531-550)

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Reviewer #1 Comment 9. Carbonyl index is sometimes used as a measurement of photochemical ageing, but it is cumbersome to assume that it will systematically increase during long-range transport, which can last hours or a few days, depending on the atmospheric circulation.

Besides Fig. 5 is non-conclusive and does not support authors' opinion. Additionally, I don't see any peak in the 1735 cm -
1 region in the rayon spectrum of Fig. 2, where C=O stretching band should be visible.

Response:
Thanks very much for pointing this out.It is true that the carbonyl index will not linearly increase with latitudes alteration during long-range transport.Therefore, when analyzing environmental samples, a large sample size is important to indicate a general trend of the carbonyl index.Moreover, the carbonyl index is a better indicator of weathering for polymers whose initial polymer structure does not contain C=O groups.For instance, polyester contains carbonyl groups and consequently any newly formed carbonyl signals attributed to polymer aging may not be easily distinguishable from the background (Li et al. 2023).Therefore, the carbonyl index alteration has not been observed for polyester fibers after UV weathering (Pinlova and Nowack 2023).For the two reasons mentioned above, we chose to focus on rayon for a cross-latitude carbonyl index alteration analysis, which had a large number of microplastics consistently recovered in samples across all latitudes and the material does not contain C=O in its pristine form.
We agree that the previous Figure 5 was indeed non-conclusive because we only obtained a general trend with a limited number of microplastics.However, we did find that the degree of rayon aging at low latitudes was significantly lower than that at higher latitudes.The rayon particle shown in Figure 2 is an example collected at a low latitude.As rayon itself does not have a C=O group in its pristine form, the peak at 1735 cm -1 is not quantifiable for this fiber.We have further supplemented the diagram of rayons' spectra comparison between low and high latitudes, where the change in the C=O position (at 1735 cm -1 ) is more clearly shown (Figure S15).
Changes to the Supporting Information:

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index is a better indicator of weathering for polymers whose initial polymer structure does not contain C=O groups, and it would not linearly increase with latitudes alteration during long-range transport, we chose to focus on rayon for a crosslatitude carbonyl index alteration analysis, which was frequently identified in samples collected.Furthermore, rayon does not contain C=O groups in pristine form.The C=O peak (at 1735 cm -1 , blue lines) is more clearly shown for rayon collected at higher latitudes, and the area under 1800 -1670 cm -1 (red line intervals) was deemed as the absorbance area (A1) of the carbonyl (C=O) group, and the area under 1500 -1390 cm -1 was deemed as the absorbance area (A2) of the reference (CH2) group in Eq.3.
Reviewer #1 Comment 10.In sum, although the authors presented interesting results, neither their novelty nor the depth of discussion is sufficient to gran publication in a top journal like NC.

Response:
We thank the reviewer for the critiques provided on several aspects of this study, which allowed us to improve our manuscript.
We have further highlighted the novelty in the revised manuscript, in addition to providing clarity to the questions posed by the reviewer.A summary of the novelty is detailed in the response to this reviewers' first comment, and we hope to have satisfied the other scientific queries by providing additional information and contextualization.We believe the paper is greatly strengthened with this additional information, more in-depth discussion, and clarity on data quality and interpretation.
End of responses to Reviewer#1

Reviewer #2 Comment 1. Dear authors, you present noteworthy results of the presence of MPs in the atmosphere across the Southern hemisphere. your work is of significance to the field of atmospheric MP research. Some information in the methods and the subsequent use of the acquired data does not always support the conclusions and claims, with additional
input and revision needed?Some flaws in the data analysis, interpretation and conclusions are pointed out below in more detail, requiring revision.

Response:
Thank you for your positive comments!According to your detailed suggestions, we have added more information to the methods and substantially enhanced the interpretation and discussion of the results.

Reviewer #2 Comment 2. Line 104: Please also see comment on method chapter, but the described use of only 30% of 1/4th of the filter results in 1.4 and less MPs per sample. The reanalysis of 100% of the 1/4 of the filter is strongly advised
for all samples inside the MBL.

Response:
Thank you very much for your advice.The reanalysis of 100% of the ¼ of the filters inside the MBL has now been performed and the results are included in the revised version of the manuscript.During manuscript revision, we reanalyzed 14 filter samples where we had previously only measured 30% of the randomly selected particles on the filter.Comparing the results of both analyses, the concentrations of both MPs and non-plastic particles between sub-sampling (1 st analysis) and the full analysis (2 nd analysis) were similar.Further explanation has been added (see Text S2) and the figures have subsequently been updated.

Text S2 Atmospheric particles concentrations comparison between the sub-sampling and full analysis
Atmospheric particles were collected onto Whatman quartz fiber filters (20.3 cm × 25.4 cm) at each site.Typically, for a sampling duration of 48 h, this would consequently lead to sampling a volume of 3456 m 3 on each filter.For sample analysis, one fourth of each filter was used for particles characterization and quantification according to a previous airborne particulate study (Moch et al. 2020).Two morphologies were included here, namely fibers and fragments, and two rounds of analysis were conducted, as detailed below.

st analysis:
As the fibers number concentrations decreased for more southerly latitudes, we found that 14 samples in the more northerly sample locations had too many fibers on one fourth of the filters to easily measure (number of fibers exceeded one hundred).
To facilitate the identification and quantitation, all fibers were observed and collected under the microscope and the total numbers were counted.Then, we randomly selected 30% of the fibers to identify their composition chemistries.Therefore, of the 26 sampling sites, we identified all fibers for 12 sampling sites (i.e., southern sites) and 30% of the fibers for 14 sampling sites (i.e., northern sites), and adjusted the final fiber number concentrations and chemical identification reported accordingly during the 1 st analysis.Fragments in all 26 samples were counted and chemical compositions were identified across the entire expedition. R11 2 nd analysis: For method verification, we used the backup samples (another one fourth of the filters) during the 2 nd analysis for a full scan to prove the robustness of this sub-sampling approach (i.e., only using 30% of the fibers in the 1 st analysis) by comparing these values with the entire analysis (100% of fibers analyzed in the 2 nd analysis).The impacts of sub-sampling a filter versus identifying the entire sample did not have large discrepancies in this instance (Table S6, Figure S17).Here, the fiber concentrations obtained between sub-sampling and the full analysis were similar, when scaled for the proportion of the filter which was analyzed.Beyond our specific study, this may be useful for other researchers in the future to assess whether a full scan needs to be performed or only a sub-section analysis is sufficient if situations where a large number of microplastics are recovered.However, we appreciate that by nature microplastics contamination can be heterogeneous, and so an assessment of the goodness of fit of subsampling should be considered in any study which does not measure the entire particle distribution in a sample.
During both the 1 st and the 2 nd analysis, all fragments (100%) on filters were identified, and a comparison of the fragments concentrations indicates that there is no significant difference between the two measurements for MPs fragments and nonplastic fragments according to t-tests (Table S7, Figure S18).

Table S6
Comparison of MPs and non-plastic fibers concentrations between an entire analysis (100% particles were fully scanned) and a sub-sampling analysis (30% randomly selected particles on the filter) for 14 samples.
Latitude MPs Fibers a MPs Fibers b Non-plastic Fibers a Non-plastic Fibers b

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Figure 3 Changes in the vertical component of the updraft wind speed which can carry microplastics with different lengths and densities.L is the length of a particle and 2r is its diameter or width; and L is defined to be always larger than 2r.The smaller of the L/2r ratio is, the shape of the particle is closer to a fragment; whereas the larger of the L/2r ratio is, the shape of the particle is closer to a fiber.%p indicates the density of a plastic particle.

Rayon is a semi-synthetic polymer, consisting of cellulose. The discussion of the ability of the FTIR to distinguish between natural cellulose fibres and rayon is missing. As a consequence, the inclusion of data on rayon should be limited to general
descriptions rather than detailed data evaluation and discussion.

Response:
Thanks very much for the comments.We have supplemented the discussion of the ability of the FTIR to distinguish between natural cellulose fibers and rayon.
"Considering that the man-made fiber rayon is challenging to distinguish from cotton or other cellulosic materials, we have adopted two methods for its accurate identification.First, the characteristic peak at 1105 cm -1 is very distinct for rayon, and we have embedded this information into the spectra matching library according to our previous research (Cai et al. 2019).
Second, another characteristic band at 3330 cm -1 of the O-H stretching can also be utilized to distinguish rayon, where its Moreover, the inclusion of data on rayon has been limited to general descriptions rather than detailed data evaluation and discussions.We have removed the detailed description of the rayon carbonyl index along the latitudes and the original Figure 5 from the main text, and moved them into the Supporting Information.
"We observed a higher CI index for MPs collected from the MBL at higher latitudes compared to mid-low latitudes (Figure S14), and the change of the C=O position (at 1735 cm -1 ) was clearly shown (Figure S15), suggesting increased weathering during long range transport.We only compared the CI index for rayon MPs as it has the most abundant occurrence for both

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fibers and fragments (Figure 5)." (Lines 259-264) "where, A1 is the absorbance of the carbonyl (C=O) peak for MPs rayon fibers and resin fragments, and A2 is the absorbance of a reference peak (CH2) for these MP samples.Detailed area bands were depicted in Text S4 and Figure S15.Here, A1 area bands used for rayon and resin were of 1700-1570 cm -1 and 1800-1650 cm -1 , respectively; and A2 area bands used for rayon were of 1500-1390 cm -1 and 1500-1350 cm -1 , respectively."(Lines 661-666)

Reviewer #2 Comment 5. Line 209 ff
Since the weathering evaluation was only done with rayon with its challenges to be correctly distinguished from natural cellulose, and rayon being the almost only polymer with a considerable number of CH3-and C=O groups potentially formed, this part is highly speculative and not supported by the available data.Please remove.

Response:
As mentioned in the response to the above Comment 4 from this reviewer, we have deleted the detailed description of the rayon carbonyl index along the latitudes and the original Figure 5 in the main text and moved them into the Supporting Information.Though we observed the formation of C=O groups of rayon MPs collected at high latitudes, this information was only provided in the Supporting Information (Figure S14, Figure S15, and Text S4).Further discussion on this point has also been included in Text S4).

Text S4 Carbonyl index calculation.
It is true that the carbonyl index will not linearly increase with latitudes alteration during long-range transport.Therefore, when analyzing environmental samples, a large sample size is important to indicate a general trend of the carbonyl index.
Moreover, the carbonyl index is a better indicator of weathering for polymers whose initial polymer structure does not contain C=O groups.For instance, polyester contains carbonyl groups and consequently any newly formed carbonyl signals attributed to polymer aging may not be easily distinguishable from the background (Li et al. 2023).Therefore, the carbonyl index alteration has not been observed for polyester fibers after UV weathering (Pinlova and Nowack 2023).For the two reasons mentioned above, we chose to focus on rayon for a cross-latitude carbonyl index alteration analysis, which has a large sample number and the material does not contain C=O in its pristine form.We have further supplemented a diagram of rayon's spectra comparison between low and high latitudes (Figure S15), where the C=O peak (at 1735 cm -1 , blue lines) was more clearly shown for rayon collected at higher latitudes, and the area under1800 -1670 cm -1 (red line intervals) was deemed as the absorbance area (A1) of the carbonyl (C=O) group, and the area under 1500 -1390 cm -1 was deemed as the absorbance area (A2) of the reference (CH2) group in Eq 3.

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Figure S15 Rayon micro-FTIR spectra comparison between samples collected from low (A-B) and high (C-D) latitudes.As the carbonyl index is a better indicator of weathering for polymers whose initial polymer structure does not contain C=O groups, and it would not linearly increase with latitudes alteration during long-range transport, we chose to focus on rayon for a cross-latitude carbonyl index alteration analysis, which was frequently identified in samples collected.Furthermore, rayon does not contain C=O groups in pristine form.The C=O peak (at 1735 cm -1 , blue lines) is more clearly shown for rayon collected at higher latitudes, and the area under 1800 -1670 cm -1 (red line intervals) was deemed as the absorbance area (A1) of the carbonyl (C=O) group, and the area under 1500 -1390 cm -1 was deemed as the absorbance area (A2) of the reference (CH2) group in Eq.3.

Reviewer #2 Comment 6. Line 439:
Please add also metric dimensions, since all other sizes are given in metric units.

Response:
Thanks so much for this kind reminder.We have converted the metric dimensions into metric units as follows.

Response:
Four field blank samples (two collected along the cruise path and two collected inland) were prepared from filters mounted in the HVAS with an air pump flow rate set to 0. As the filters can directly have contact with the surrounding air, the period of time was set as "30 s" to avoid potential extra contamination.We have revised the manuscript to include this point.
"Furthermore, four field blank samples (two collected along the cruise path and two collected inland) were prepared from filters mounted in the HVAS with an air pump flow rate set to 0. To avoid the potential contamination from the surrounding R17 air environment, the period of time for the blank filters was set as "30 s".For the field blanks, no MPs were found."(Lines

Reviewer #2 Comment 8. Line 461 ff:
Please extend on the information on how the filter was placed, removed from the sampler and stored prior analyses.

Response:
Thanks for the question and the opportunity to further clarify the methods we used in this study.In the revised manuscript, we have provided additional details about the operation process.
"For mounting the filters, the buckles of the upper placing plate were unscrewed after opening the upper cover of the HVAS, and then a filter membrane was laid flush on the lower plate.Then the buckles were re-tightened, and the upper cover of the HVAS was closed.The entire process was generally completed within about 30 s, reducing the chance of airborne contamination.Note that the filter has one rough surface and one smooth surface, and atmospheric particles were always collected on the rough surface.Typically, the pump of the high-volume air sampler (TISCH Environmental, USA) was running at a constant flow rate, 1.2 m 3 min -1 , and for a sampling duration of 48 h which would consequently lead to sampling a volume of 3456 m 3 .The flow rate of the pump was calibrated when manufactured in the factory.The airflow rate of the HVAS was relatively constant, at 1.2 m 3 min -1 , and a sampling duration of 2-3 days for each sample led to typical sampling air volumes of ~3000-4000 m 3 .Typically, aAtmospheric particles along the 2-3 days' cruise path, covering approximately 2-4 degrees of latitude, were collected on one filter which was considered one aggregate sample.
A wind direction sensor was employed to control the HVAS to avoid potential contamination from the vessel, so consequently only air masses from a sector ~120 o left and right of the central line of the vessels' path was sampled.After sampling, the upper cover of the HVAS was opened and buckles of the upper placing plate were unscrewed, then the filter was removed by pre-cleaned stainless tweezers from the sampler.After sampling, individual Individual filters were kept separate, folded, wrapped in aluminum foil, placed in zip-loc bags, and stored in the dark under freezing conditions (-20 °C) until particle characterization processing began."(Lines 531-552) Reviewer #2 Comment 9. Line 493:

Please specify if the analysed MPs represented 30% of the 1/4 of the filter? Can you show that this is representative for the whole filter also from filters from inside the MBL with a low MP count?
Response: Thank you very much for this kind advice.The reanalysis of 100% particles of the ¼ of the filters inside the MBL has now been performed and the results are included in the revised version of the manuscript.During the manuscript revision, we reanalyzed fourteen filter samples where we had previously only measured 30% particles in the filter area.
We used the corresponding backup samples for a full scan to better compare this sub-sampling (30%, as initially done) with an entire analysis (100%, now performed).In the updated figures and tables, we consequently show the impact of subsampling a filter versus measuring the entire sample (Table S6, Figure S17).In this case, the number and characteristics of particles which were obtained between sub-sampling and the full analysis were similar, when scaled for the proportion of the filter which was analyzed.Beyond our specific study, this may be useful for other researchers in the future to assess if an

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entire filter needs to be analyzed or only a sub-section, since if a sub-section is sufficient, this would consequently save time and effort across the analytical quantification step.We have updated Figure 1 and other relevant figures in the revised manuscript to include the results from both analysis methods, where the same trends for fibers and fragments were observed independent of the extent of filter analysis.Detailed explanation has also been added in the revised manuscript and Text S2.
"For fibers, we found that 14 samples in the more northerly sample locations had too many fibers on one fourth of the filters to easily measure (number of fibers exceeded one hundred).To facilitate the identification and quantification, all fibers were observed and collected under the microscope and total numbers were counted.Then, we randomly selected 30% of the fibers on these 14 filters to identify their composition chemistries.For this method verification, we used the backup samples (another one fourth of the filters) during the 2 nd analysis for a full scan to prove the robustness of the sub-sampling approach (i.e., only using 30% of the fibers in the 1 st analysis) by comparing these values with the entire analysis (100% of the fibers analyzed in the 2 nd analysis) (see Text S2). when the number of fibers in a sample exceeded 100, one-third of the randomly selected items were representative of the entire fiber population collected on a filter (according to the results of the first twelve sampling sites examined)."(Lines 606-618)

Text S2 Atmospheric particles concentrations comparison between the sub-sampling and full analysis
Atmospheric particles were collected onto Whatman quartz fiber filters (20.3 cm × 25.4 cm) at each site.Typically, for a sampling duration of 48 h, this would consequently lead to sampling a volume of 3456 m 3 on each filter.For sample analysis, one fourth of each filter was used for particles characterization and quantification according to a previous airborne particulate study (Moch et al. 2020).Two morphologies were included here, namely fibers and fragments, and two rounds of analysis were conducted, as detailed below.

st analysis:
As the fibers number concentrations decreased for more southerly latitudes, we found that 14 samples in the more northerly sample locations had too many fibers on one fourth of the filters to easily measure (number of fibers exceeded one hundred).
To facilitate the identification and quantitation, all fibers were observed and collected under the microscope and the total numbers were counted.Then, we randomly selected 30% of the fibers to identify their composition chemistries.Therefore, of the 26 sampling sites, we identified all fibers for 12 sampling sites (i.e., southern sites) and 30% of the fibers for 14 sampling sites (i.e., northern sites), and adjusted the final fiber number concentrations and chemical identification reported accordingly during the 1 st analysis.Fragments in all 26 samples were counted and chemical compositions were identified across the entire expedition.

nd analysis:
For method verification, we used the backup samples (another one fourth of the filters) during the 2 nd analysis for a full scan to prove the robustness of this sub-sampling approach (i.e., only using 30% of the fibers in the 1 st analysis) by comparing these values with the entire analysis (100% of fibers analyzed in the 2 nd analysis).The impacts of sub-sampling a filter versus identifying the entire sample did not have large discrepancies in this instance (Table S6, Figure S17).Here, the fiber concentrations obtained between sub-sampling and the full analysis were similar, when scaled for the proportion of the filter which was analyzed.Beyond our specific study, this may be useful for other researchers in the future to assess whether a full scan needs to be performed or only a sub-section analysis is sufficient if situations where a large number of microplastics are recovered.However, we appreciate that by nature microplastics contamination can be heterogeneous, and so an assessment

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of the goodness of fit of subsampling should be considered in any study which does not measure the entire particle distribution in a sample.
During both the 1 st and the 2 nd analysis, all fragments (100%) on filters were identified, and a comparison of the fragments concentrations indicates that there is no significant difference between the two measurements for MPs fragments and nonplastic fragments according to t-tests (Table S7, Figure S18).
Table S6 Comparison of MPs and non-plastic fibers concentrations between an entire analysis (100% particles were fully scanned) and a sub-sampling analysis (30% randomly selected particles on the filter) for 14 samples.
Latitude MPs Fibers a MPs Fibers b Non-plastic Fibers a Non-plastic Fibers b

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density was used here?

Response:
Thanks very much for the kind reminder.We have added the references for both fiber mass and fragment mass calculations and their corresponding references as follows.
"We used the cylinder model (Simon et al. 2018) and the column model (Koelmans et al. 2020) to estimate the surface area and volume (mass) values of fibers and fragments (details see Text S3).Additionally, an approximation by assuming the length-to-width ratio equates to the width-to-height ratio (L/W=W/H) was also applied for fragments (Mintenig et al. 2020, Simon et al. 2018).However, it is important to note that this simplified approach to suggest surface area and volume (mass) likely represents a low estimate, as it does not take into account additional surface roughness and cracks which are likely to be present on environmental microplastics, subsequently increasing the actual surface area."(Lines 631-638) For the microplastic fibers, we measured the projected width and length of particles utilizing ImageJ.The width was equivalent to the diameter of the bottom surface of a cylinder, the length was equivalent to the length of the cylinder, and calculated the mass according to Equation 1.For the microplastic fragments, we measured the projected length and width, utilizing ImageJ directly, projected the height using L/W=W/H, and calculated the mass concentration (MC) and surface area (SA) according to Equation 2.
where n is in the total number of fibers in the sample, i; vi is the sampled air volume, f is the fiber void fraction (40%) (Simon et al. 2018), because airborne fibers become looser than their original states (Figure S16), and the value of f=0.4 was used here.R and L were the fiber diameter and length as calculated by the Image J software according to the top-view projection images, respectively.% is the average density of primary fibers collected in this survey (Table S4).The above-detailed explanation and calculation was included in the revised Supporting Information (Text S3).
The density used here was the average density of primary fibers and primary fragments collected in this survey.For instance, rayon and polyester were the dominant MPs fibers detected, and rayon and epoxy resin were the dominant MPs fragments detected.Therefore, the average densities for MPs fibers and MPs fragments were calculated based on these primary MPs densities and their composition percentages of 1471 kg/m 3 and 1404 kg/m 3 , respectively.Further, we explained the density selection more clearly in the revised manuscript as follows.
"% is the average density for fragements average density of primary fragments collected in this survey (Table S4)." (Lines

Reviewer #2 Comment 12. Line 523:
A reference for this method is missing.How was that method verified?.State that this a method only applicable for rayon and other polymers forming C=O from CH3-groups. Response:

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Thank you very much for your valuable suggestion!It is true that this method is applicable for rayon but probably not suitable for other polymers and plastics, such as polyester, where the pristine polymer already contains C=O groups.We have added relevant literature that supports this, as explained in further detail below.
First, rayon (a man-made cellulose) is challenging to distinguish from cotton or other cellulosic materials using spectroscopic method.As this was also an issue that occurred during our field investigation, our group has developed a method to accurately distinguish rayon from cotton (Cai et al. 2019).The characteristic peak at 1105 cm -1 is very distinct for rayon, which can be applied to distinguish man-made rayon from natural cotton as well.Therefore, this is an important feature for polymer identification in this and we have embedded this information into the spectra matching library.
Second, it is true that the carbonyl index will not linearly increase with latitudes alteration during long-range transport.
Therefore, when analyzing environmental samples, a large sample size is important to indicate a general trend of the carbonyl index.Moreover, the carbonyl index is a better indicator of weathering for polymers whose initial polymer structure does not contain C=O groups.For instance, polyester contains carbonyl groups and consequently any newly formed carbonyl signals attributed to polymer aging may not be easily distinguishable from the background (Li et al. 2023).Therefore, the carbonyl index alteration has not been observed for polyester fibers after UV weathering (Pinlova and Nowack 2023).For the two reasons mentioned above, we chose to focus on rayon for a cross-latitude carbonyl index alteration analysis, which has a large sample number and the material does not contain C=O in its pristine form.We have further supplemented a diagram of rayons' spectra comparison between low and high latitudes, where the change of the C=O position (at 1735 cm -1 ) is more clearly shown (Figure S15, Text S1).We also added an explanation in the revised manuscript as follows.
"We chose rayon for a cross-latitude carbonyl index alteration analysis, because rayon (man-made cellulose) which has a large detection rate, does not contain C=O itself, and has been verified to be photochemically degraded by near UV and visible radiation and form oxidized groups (carbonyls and carboxyls) (Ahn et al. 2019, Egerton and Shah 1968, Pinlova and Nowack 2023)."(Lines 264-268) Figure S15 Rayon micro-FTIR spectra comparison between samples collected from low (A-B) and high (C-D) latitudes.As

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the carbonyl index is a better indicator of weathering for polymers whose initial polymer structure does not contain C=O groups, and it would not linearly increase with latitudes alteration during long-range transport, we chose to focus on rayon for a cross-latitude carbonyl index alteration analysis, which was frequently identified in samples collected.Furthermore, rayon does not contain C=O groups in pristine form.The C=O peak (at 1735 cm -1 , blue lines) is more clearly shown for rayon collected at higher latitudes, and the area under 1800 -1670 cm -1 (red line intervals) was deemed as the absorbance area (A1) of the carbonyl (C=O) group, and the area under 1500 -1390 cm -1 was deemed as the absorbance area (A2) of the reference (CH2) group in Eq.3.
End of responses to Reviewer#2

Response:
We truly appreciate the positive feedback provided by the reviewer.Significant progress has been made in data analysis and adapting the interpretations, with the incorporation of comprehensive discussions that address the insightful comments and suggestions provided by all reviewers.Overall, the quality of this paper has been significantly enhanced upon revision.

Reviewer #3 Comment 2. I summarize my comments and suggestions here below:
The main weakness to the data presentation is the lack of any temporal reference, as there is no info on when the observations have been collected.It is only mentioned on the method paragraph that the campaign took place in the "2019-2020 season".This is very vague, and it doesn't allow for any consideration on the atmospheric processes that may have influenced the MP transport.This information is also missing from the data files provided in the attachment.

Response:
Thanks for this comment.Following the reviewers' suggestion, a table summarizing the detailed sampling information has now been included in the Supplementary Information (Table S1).
Please find the whole table in the next page. R26

Response:
Thanks very much for the comments.Yes.The different samples collected at different latitudes related to different seasons.
Although the samples were collected in different seasons, our observations in the high southern latitudes suggest that there is no significant difference for MPs and non-MPs concentrations between November (A12-A15 and A22-A24) and March (A16-R27 A21) (independent-samples t test; p>0.05).Specifically, we plotted a comparison figure between the close sampling locations within latitudes (60 °S-65 °S) including A16-A21 (March) and A13-A15,22 (November).Thus, the differences in MPs and non-plastic particles concentrations of different latitudes are unlikely to be dominated by the varied sampling seasons, and the atmospheric transport efficiency would be responsible for the spatial patterns of MPs.

Reviewer #3 Comment 4. Lines 84-85: The authors suggest that there is a decrease in MPs fragment concentration from north to south, as if there is a linear relationship between latitude and microplastic presence in the atmosphere (hypothesis also reinforced by the use of the linear regression analysis in figure 1) which do not seem to be justified. What is this hypothesis based on?
Response: Thanks very much for the kind questions.In our research, continental MPs are hypothesized to be the main sources for Southern Ocean atmospheric MPs, and we used the linear regression model to fit the curves according to the suggested relationships between particle diameter and sedimentation velocity as explained below and in Text S1: R28 "Hicks et al. found that the dry settling velocity of atmospheric aerosol particles is directly proportional to friction velocity (Hicks et al. 2016); and Vong et al. further obtained the formula of dry settling velocity after dimensionless treatment with friction velocity (Vong et al. 2010) as follows.
Vd is the settling velocity and has a unit of cm s _* , C is a numerical coefficient, Uf is the friction velocity and has units of m s _* , L is the Monin-Obukhov length and has units of m, and D is the diameter of the particle and has units of µm.
Based on the above equation (Eq S3), we assume for a specific-sized particle, its Vd is close to a constant.Thus, we hypothesize that the relationship between distance and MPs concentrations follows a linear relationship.We also suppose that the linear decrease trend may not due to the difference in latitude, but the distance from populations and an explanation has been supplemented in the manuscript.
"Continental MPs are hypothesized to be the main source for Southern Ocean atmospheric MPs, and we used the linear regression model to fit as explained in Text S1.We used latitude as a proxy for the impacts of anthropogenic emissions, which are expected to decrease from north to south along the study transect.Thus, the linear decrease trend for fragments is not due to the difference in latitude, but rather the distance from populations.The majority of sources (i.e., land from populated urban centers in Southern Asia and Oceania) in the Southern Ocean are located to the north.However, this latitude trend may not be applicable to other regions, such as an equivalent latitude difference across a continent, because inputs from population centers across the latitudes would be continuous.

Response:
Thanks very much for the kind comments.As the reviewer pointed out, the back-trajectory analysis can provide an important message for the airmass directions and potential land sources.In the revised manuscript, we have included additional sampling points for back-trajectory analysis and have incorporated a more comprehensive examination of the vertical uplift of the air masses.Furthermore, we have expanded the discussion section to provide a more thorough analysis.More details are provided in the point-by-point responses to similar comments on this topic below.

Response:
A good point!Yes, in the trajectory analysis the vertical uplift of the air masses were not initially considered.The vertical upward wind (1-1.5 m s -1 ) can occur fairly frequently in convective updrafts (e.g., Frank et al. 2013).In this case, the particles near the surface would be uplifted and released into the atmosphere.In addition, it has previously been reported that large dust particles of up to 0.45 mm in diameter can be suspended and transported to locations more than 3000 km from their source (van der Does et al. 2018).The density of dust particles is larger than those of plastic particles, and thus it is expected that the MPs can also be suspended from the surface and transported far away (Bullard et al. 2021).In the revised manuscript, we have explained this as follows.
"The transport distance of particles is intricately linked to their aero sedimentation velocity and vertical upward wind.For compact MPs fragments with effective radius, r, OG BPPROXJMBTFLY /) ] 1) `M BNE B EFNSJTY OG [*'-H DM -3 (e.g., PVC), its aero sedimentation speed, U, is estimated to be between 1.3 -1.5 m s -1 .That is, for an updraft wind with a vertical component greater than approximately 1.5 m s -1 , fragments of similar or smaller masses will become airborne and carried by the wind.
Similarly, the lower %p corresponds to lower U; e.g., polyethylene (0.91-0.94 g•cm -3 ) and polystyrene (0.96-1.05 g•cm -3 ), their estimated sedimentation speed is approximately 1.0-1.2m•s -1 .As reported, the vertical upward wind (1-1.5 m s -1 ) can occur fairly frequently in convective updrafts (e.g., Frank et al. 2013).In this case, the particles near the surface would be uplifted and released into the atmosphere."(Lines 147-157) In terms of the air mass transport over the continents, we calculated the backward ensemble trajectory of samples that are influenced by the continental sources to show changes in elevation during air mass transport (Figure R1).In this case, we took four samples as an example.It is clearly shown that the air mass can be near the surface or in the planetary boundary layer (<~2000m) when travelling over land.Therefore, we can anticipate that MPs found in our samples are mainly from the continental sourced air.

Response:
Thanks for the comments, and 7 days is the length of the trajectories computation back in time.Previously published reports suggest that the residence time for MPs particles in the atmosphere may vary between 1 and 156 h (Brahney et al. 2021), and usually the backward trajectories were run for 6-7 days to show the full range of possible sources (Aves et al. 2022).These

Response:
We thank the both reviewers very much for their time in evaluating our work.Also, we appreciate their positive comments on our revised version.

Figure 1 :
Figure 1: It is not clear to me how should I read the ** symbols over the connected histograms.Why is in panel A connecting all latitudes with Antarctica and in panel B the Northern latitudes with all the others?What does the purple horizontal bar represent?

Figure S15
Figure S15Rayon micro-FTIR spectra comparison between samples collected from low and high latitudes.As the carbonyl #3 Comment 1.The paper analyses the microplastic concentrations in the atmosphere over the marine surface collected during a cruise in the Southern Ocean.The study gives important results on the field, providing with a new relevant dataset of observations in a region that was otherwise not well explored yet.While, for the relevance of the resultsand content of the paper I would encourage a final publication of this work on Nature Communications, I think some major revisions on the way the data, the analysis, and some of the conclusion are presented, are necessary.

Figure
Figure S4 Seasonal factors have limited influence on MPs and non-plastic particle number concentrations.A comparison between sampling locations A16-A21 (March, austral autumn) and A13-A15, 22 (November, austral spring) within latitudes (60°S-65°S).ns: indicates no statistically significant differences between the two groups (p>0.05) according to t-tests.
back-trajectory analysis, which seems to carry an important message in this work (i.e. that the northern latitudes samples are related to a meaningful influence of land sources with respect to the southern ones), looks not very robust.

Figure S5
Figure S5 Backward trajectories of all samples collected in marine boundary layer.Detailed information of samples shown in each individual panel are provided in TableS1.

Figure R1
Figure R1Backward ensemble trajectories of the samples that are influenced by the continental air mass, as well as the elevation of air mass during the transport along individual trajectories.Panels (a), (b), (c), and (d) correspond to the samples A02, A06, A07, and A08 (TableS1), respectively.
have any further comments and I am happy with the revisions Reviewer #3 (Remarks to the Author): The work is bringing new and relevant knowledge about the problem of atmospheric microplastic and its possible sources.Since the reviewed paper has noticeably improved, and the research results are now presented in a more robust and sound way, I do support the publication of the reviewed manuscript without further revisions.bringing new and relevant knowledge about the problem of atmospheric microplastic and its possible sources.Since the reviewed paper has noticeably improved, and the research results are now presented in a more robust and sound way, I do support the publication of the reviewed manuscript without further revisions.

Table S3
Summary of airborne microplastics concentrations collected with air samplers.

Table S1
Detailed information on the aerosol sampling dates and locations in the marine boundary layer and inland Antarctica.Temperature of air was obtained from the shipboard automatic weather station.
a the mean values of individual sampling voyage legs.